To me, the tip is the triangular shaped point. It's kind of hard to say for certain, but for me, the top of that triangle touches the alveolar ridge.
It shouldn't touch your teeth, because then it's going to start sounding more like a 'th' sound.
It should basically be the same position as a 't'. The only difference (in English) between a /t/ and a /d/ sound is that the /d/ is voiced, while the /t/ isn't. The position is the same.
Check out this website - you'll find the /t/ and /d/ phonemes under the 'stop' tab.
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